In modern communication systems, the information items to be transmitted are usually coded both in terms of the phase and in terms of the amplitude of a signal. As a result, it is possible to achieve significantly greater data transmission rates than in the case of conventional types of modulation which use pure amplitude or phase modulation, respectively. Examples of such types of modulation are the PSK modulations (Phase Shift Keying) such as a π/4-DQPSK, 8-DPSK or 8-PSK modulation, but also a Quadrature Amplitude Modulation (QAM). They are referred to as digital modulation types, in contrast to analog amplitude or frequency modulation.
FIG. 6 shows a constellation diagram for an 8-PSK modulation. In this case, the x axis represents the first, real component I and the y axis represents the second, quadrature component Q. The information to be transmitted is coded by a value pair i, q, depending on its content in one of the points represented. A value pair i, q is referred to as a symbol. In the example illustrated, one symbol codes a total data content of 3 bits in the case of an 8-PSK modulation. It is evident that the amplitude of the i and q values changes over time, depending on the data content to be coded. Therefore, an 8-PSK modulation is referred to as a modulation type exhibiting non-constant envelope modulation. The 8-PSK modulation is used for example for the GSM/EDGE mobile radio standard.
In addition to representing a symbol by means of a value pair i, q, it is possible to specify the symbol in terms of its phase φ and its amplitude r. The two representations in I/Q notation and rφ notation are synonymous.
Accordingly, in addition to I/Q modulators, polar modulators can also be used for the transmission of modulated signals. While I/Q modulators process I and Q signals for a modulation, polar modulators modulate the phase and the amplitude. The functioning for an I/Q modulator is described in Behzad Razavi, “RF Micro-electronics”, Chapter 3.3.3.
One example of a known polar modulator is shown in FIG. 5. The information items to be transmitted are present as digital data ak and are conditioned in a coder circuit and a further circuit. The digital data are then converted into their phase value φ(k) and also into an amplitude value r(k) in the circuit 900. The phase information φ(k) is fed to a phase locked loop PLL. It is used to modulate the output signal of the phase locked loop PLL in accordance with the phase-coded information. A phase-modulated output signal φ(t) is thus present at the output of the control loop PLL. At the same time, the amplitude information r(k) is fed to a digital-to-analog converter DAC which converts the amplitude information r(k) present in digital fashion into a temporal analog signal. The analog amplitude modulation signal r(t) is fed to a mixer via a low-pass filter. The phase-modulated signal is combined with the amplitude modulation signal in said mixer.
The requirements made of the final mixer stage are problematic in the case of this solution. Said mixer stage should have a sufficiently high linear transfer response in order to comply with the large amplitude range required in many mobile radio standards.
In the case of a nonlinear transfer response of the mixer, amplitude or phase distortions dependent on the amplitude modulation signal r(t) may occur. Such distortions are referred to as AM/AM or AM/PM distortions. The distortion generates data errors and the frequency spectrum of the signal that is output changes.
FIG. 4 shows the frequency spectrum as a function of an amplitude/phase distortion. It is evident that even a slight frequency offset of 1°/dB infringes the spectral mask of the GSM/EDGE mobile radio standard as illustrated in FIG. 4. It is therefore necessary to minimize distortions.
The embodiment of a polar modulator that is known in FIG. 5 leads to a high space requirement for the mixer when account is taken of the requirements made of a very linear response. Moreover, such a polar modulator cannot be implemented in novel CMOS technologies with low supply voltages in the range of 1.5 V to 2.5 V, since linearity is not ensured there.